Synthetic polyamide filaments of high impact strength and process of making same



' preferred.

Patented Oct. 13, 1942 UNITED STATES PATENT OFFICE SYNTHETIC POLYAMIDE FILAMENTS OF HIGH IMPACT STRENGTH AND PROCESS OF MAKING SAME Willard E. Catlin, Wilmington, Del., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del, a corporation of Delaware I No Drawing. Application April 3, 1940, Serial No. 327,735

7 Claims.

This invention relates to the fluid treatment of filaments for the purpose of enhancing their impact or knotted impact strength. The invention also relates to articles of manufacture in or with which are incorporated monofil or multifil strands or strings consisting of or formed g-from filaments so treated, or fabrics comprising the same. More particularly, the invention re lates to the manufacture of strings of the kind used in such stringed instrument or implements as are typified by rackets and viols, or as, in common with them, call for strings that not only possess good tensile strength but also possess a high degree of impact, or knotted impact, strength. Still more particularly this invention relates to the fluid treatment, for the said purpose, of filaments comprising fiber-forming synthetic linear polymers, especially those of the classes described in U. S. 2,071,250, 2,071,251,

,,,2,071,253, 2,130,523, 2,130,948, of which classes the synthetic linear polyamides usually are to be The most suitable strings for tennis, squash or badminton rackets, and the like, heretofore have ubeen prepared from the gut of sheep and hogs.

,linear polymers has been suggested for the aforesaid purposes, especially in view of the excellent tensile strength and resiliency of such filaments,

as a class, but those which have been prepared and conditioned in merely the conventional manner nevertheless have failed to prove satisfactory. Poor resistance to shock of sudden impacts has been responsible for such failure.

This invention accordingly has as an object the improvement of the said resistance. A further object is the improvement of the resistance 1 of the said filaments to the fatigue of repeated impacts. A still further object is the enhancement of the moisture and weather resistant properties of the filaments. Additional objects will appear hereinafter. The first two of the said objects may be accomplished, in accordance with the invention, by

heating the said filaments, after they have been oriented as described hereinafter, in the presence of 'a relatively mild nonsolvent swelling agent, while permitting free shrinkage to take place during the said heating. The duration of the treatment must be terminated, however, be- -fore either the tensile strength or the elongation at break has been sacrificed unduly. In other words, I have discovered that the aforesaid shrinkage, consequent on thefilaments being in the relaxed condition during the treatment, not only is accompanied by a gradual but marked increase in the impact and knotted strengths of the filament, as well as its general toughness, but at the same time entails a gradual and substantial decrease in tensile strength and an increase in the elongation at break. As a rule, however, a decrease up to 30%, in tensile strength may be tolerated, and the elongation of the product may be as high as 50%. I, therefore, terminate my said process before changes of this magnitude have taken place, after having continued the process, nevertheless, for a period of sufficient duration to yield increases of impact strength of at least 25% and preferably at least 50%.

The said process has been'found to be particularly useful when applied to the hereinabove referred to synthetic linear polyarnides. The term synthetic linear polyamide includes those types of polymers which are derived from poly: merizable monoaminocarboxylie acids and their amide-forming derivatives (ester, halide, anhydride, and amide), those derived from the reac tion of suitable diamines with suitable dibasic carboxylic acids or the amide-forming derivatives of such acids, and those derived from mixtures of the foregoing types of reactants. On hydrolysis with strong mineral acids the polyamides revert to monomeric bifunctional polyamide-forming reactants. Thus, a polyamide derived from a diamine and a dibasic carboxylic acid yields on hydrolysis with hydrochloric acid, a dibasic carboxylic acid and a diamine hydrochloride.

As indicated in the hereinabove-cited patents, the high molecular weight synthetic linear polyamides may be formed into useful filaments which, upon application of tensile stress, are permanently stretched or cold drawn into pliable strong fibers, which show by characteristic X-ray diffraction patterns that they are oriented along their fiber axes. Orientation also may be effected by the application of compressive stress, such as that which takes place in the process of cold rolling, or by a combination of tensile and compressive stresses.

The properties 'of filaments prepared from these polyamides may depend, to a large extent, upon the conditions under which they are prepared. Filaments formed by the extrusion of the molten polymer generally are rapidly cooled or quenched, in order to impart to them enhanced toughness and pliability.

The conditions to which the filaments are subjected, after their orientation, have an effect on their properties. For instance, extensive subjection to dry heat, or to heat in the absence of a swelling medium, especially while under little tension, substantially reduces the capacity of the filament to shrink in response to the treatment of this invention; as also does extensive heating 5 even in the presence of such a medium-though often to a lesser extent-in cases where the filament is not free to shrink during such heating.

When oriented filaments prepared from the aforesaid synthetic linear polymers, but not subsequently subjected to heating under the said unfavorable conditions, are exposed to heat in the presence of water or other nonsolvent swellingagent, under such conditions that free contraction can occur, in accordance with the invention, not only does the fiber tend to shrink, but internal stresses within it tend to be relieved, and its crystal structure becomes more nearly perfect, as evidenced by X-ray diffraction patterns. Samples of oriented polyhexamethylene sebacamide thus conditioned with free contraction in saturated steam have shown as high as 3-10 fold increases in fatigue resistance to repeated impacts.

As already pointed out, however, such conditioning tends to lower the tensile strength and increase the elongation at break. Moreover, prolonged treatment at high temperatures may exert a deleterious chemical action on the particular polymer. It therefore is desirable to regulate the factors of time and temperature so as to obtain an optimum balance of impact strength, tensile strength, and elongation. Experimentation has demonstrated that tennis racket strings prepared from a synthetic linear polymer should have a tensile strength of at least 80 pounds, preferably 100 pounds, to avoid breakage during stringing, but elongations at break of less than 50%, preferably less than 35%, to facilitate stringing and avoid cold flow during storage or use, and a knot impact strength of at least 30 inch pounds, preferably higher, to avoid breakage during play.

Although satisfactory tensile values may readily be obtained with monofils or polyfils of all of these polymers by conditioning them under tension in the usual manner, conditioning with free shrinkag appears to be the only satisfactory method whereby a filament can be prepared which combines the three previously mentioned properties in such a way as to be of pronounced practical value for racket strings and the like.

The following examples involve the application of the invention to filaments formed from certain preferred polyamides, but nevertheless may be regarded as generally illustrative of the application of the invention to the aforesaid types of polymers, broadly.

EXAMPLE I A polyhexamethylene sebacamide filament having a diameter of 113 mils (0.113 inch), prepared from polymer having a melt viscosity of 2880 poises at 285 C., was soaked in water for several days, at room temperature. The filament then wa passed through hot water for a short distance and, next, directly through a die of such siz as to give a drawn filament 52 mils in diameter. The die-drawn filament was next conditioned by heating it in the relaxed condition in an autoclave, for a period of half an hour, in the presence of saturated steam and at a temperature of 151-153 0. Physical properties of the filament, before and after conditioning, respectively, are shown in the following table.

2,2os,ses

Tum: A

Physical properties of poluhexamethulene sebacamide filaments before and after conditioning 30 minutes in saturated steam at 152 0..

T il El tlo str lxth of ans 0 cage 11 sample strength at break knotted filament Pound: Percent Inch tbs. Before conditioning 125 l2 15 After conditioning 109 25 EXAMPLE II An oriented polyhexamethylene sebacamide filament was prepared by melt extrusion, with water quenching of the extruded filament. The filament then was drawn, in the manner described in Example I. The polymer in this case had a viscosity of 460 poises at 285 C. An unconditioned sample of the straight, unknotted filament registered an initial impact strength of 30 inch pounds, when a 3" length was held under a 26-lb. tension and broken in the Olsen pendulum-type impact tester. A second sample of this filament, which was conditioned by boiling in water for 1.5 hours while on a bobbin so that it was not free to contract, registered an impact strength of 46 inch pounds. But a third sample, which was conditioned similarly except that it was wrapped in a loose coil so that it could contract freely, registered an impact strength of over inch pounds.

EXAMPLE HI Polyhexamethylene sebacamid having a melt viscosity of 2275 at 285 C. was prepared in the form of an oriented filament by melt spinning under quenching conditions, followed by die drawing to a diameter of about 53 mils. .A sample of this filament was tested without having been given any conditioning treatment. A second sample was conditioned one-half hour in '10 pounds (gauge) saturated steam, whil allowed to contract freely, and tested. A third sample was given the same steam treatment, but wound on abobbin so thatit could not contract. The

temperature in each case was 156 C. The physical properties of these samples, as disclosed in the said tests, are set forth in the following table.

, TABLE B Polyheramethylene sebacamide filaments condiaaaaaes It will be noted that the sample which was conditioned in the relaxed state registered a substantial increase in knot impact strength, whereas the sample which was conditioned under tension showed an actual decrease in knot impact strength.

Eflect of conditioning temperature on the physical Properties of interpolumer filaments EXAIMPLE IV An interpolymer prepared by the condensation polymerization of a mixture of 30 arts of hexamethylenediammonium adipate and 70 parts i Break Break mun Sample and treatment (0.5 hr.) in length Diameter strength elongation Knotted Straight Percent Mile Pounds Percent 1 Original 49 97 24 10 14 2 Boiling water 14 53 104 31 19 62 3 Steam, 151bs./in. 17 54 100 40 22 70 4 Steam, 30 lbs./in. 56 87 43 31 67 6 Steam, 45 lbs/in. 23 57 83 61 73 88 6 Steam, 60 lbs/i113. 29 59 64 81 52 84 Diameter before drawing: 106 mils.

of hexamethylenediammonium sebacate, having a viscosity of 435 poises at 285 C., was extruded into a quenching bath of cold water, and congealed in the form of a filament having an average diameter of 108 mils. This filament was diedrawn to a diameter or 50 mils and five samples of the oriented material treated for one-half hour periods at several different temperatures, under conditions permitting contraction of the filament. The respective physical properties of the so treated filaments are. set out in the following table.

In the case of this polymer it appears that the optimum balance of tensile and impact strength is obtained in filaments which are conditioned for about one-half hour in saturated steam and at gauge pressures in the neighborhood of 30 lbs.

per square inch. When saturated steam is used at higher pressures (Samples 5 and 6) the time of treatment should be reduced.

Oriented filaments or ribbons prepared from other, fiber-forming synthetic linear polymers than those described above may be conditioned by this process, but the greatest field of use for Team: G

Effect of conditioning temperature on the physical properties of interpolvmer filaments Impact strength Decrease (inch pounds) in length Break strength Break Sample and treatment (0.5 hr.) elongation Knotted Straight Percent Pounds Percent Original s8 12 Boiling water Steam, l5 lbs./in. at 121 0.- Steam, 30 lbs/in. at 134 G.

Steam, lbs/in. at 144 C 88 Above 100 Above 100 Above 100 QHFWM sass In the case of the samples listed in Table C it was considered that the optimum balance of physical properties had been achieved by conditioning in boiling water; although treatment with saturated steam at 121 C. also was considered to be satisfactory. It will be noted that because of the higher temperatures employed with the fourth and fifth samples, and the consequent speeding up of the conditioning transformation, the maximum tolerable limit for break-elongation was exceeded in the 0.5 hr. treatment. At these temperatures the time of treatment for this polymer should be reduced to about 0.2-0.3 hr. Similar data, also illustrating the importance of terminating the treatment before it 7 has progressed too far, will be found in the next example, opposite Samples 5 and 6.

EXAMPLE v An interpolymer was prepared in the usual 5 manner, by the condensation polymerization of 7 the invention appears to be in connection with the said polyamides. Among them polyhexamethylene sebacamide, polydecamethylene adipamide, polydecamethylene sebacamide, and the interpolymers prepared from sebacic acid, adipic acid, and hexamethylenediamine, and from adipic acid, hexamethylenediamine and 6-aminocaproic acid or 12-aminostearic acid, are preferred types. The invention also is practiced to particular advantage with those polyamides or interpolyamides which are formed with azelaic acid or its derivatives, since very large oriented filaments can readily be prepared from such materials. The polymers of greatest utility have intrinsic viscosities 0 above 0.6, as defined in U. S. P. 2,130,948, and melt viscosities, at 285 0., above 400 poises, preferably above 1000 poises.

The polymer viscosity and the method and extent of drawing each have an important hearing on the physical properties of the final filament. For polyhexamethylene sebacamide the most desirable combination of high tensile and impact strengths, and low elongation at break, are obtained by multiple-die drawing" (explained hereinbelow) of filaments made from polymer having a viscosity of 1500 to 3000 poises at 285 C., until the diameter of the filaments has been reduced to about 0.46 that of the filaments before drawing. Conditioned filaments having satisfactory impact strengths may be obtained, however, from filaments of lower viscosity (400 to 1500 poises) which have been drawn to only 0.49-0.47 their original diameter. The drawing of medium and low viscosity polymers to a still greater extent generally results in embrittlement, if the products are conditioned with the use of high pressure steam. This embrittlement is manifested by low impact strength, and in severe cases by crazing and low tensile strength.

A reduction of the filament diameter to 0.46 of the original value by a single draft of the undrawn filament through a die will result, at times, in latent embrittlement, which will become manifest after conditioning. Such embrittlement can be avoided by multiple-die drawing." 1. e., by drawing the filament through a tandem arrangement or "nest of two or more dies of such sizes that at least half of the reduction in crosssectional area is accomplished before reaching the last die. Like results have been obtained by the use of a single die, in which thelast five or six mils of taper is more gradual than that of the usual type of die. For the best results, die surfaces must be clean and well polished at all times.

Data showing the dependence of draw ratio and polymer viscosity on the impact strength are given in the following table.

TABLE E Influence of draw ratio and viscosity on impact strength of 15 gauge polyhemamethylene sebacamz'de monofils Ratio of Knot impact y 'i g g drawn to strength of a C original conditioned diameter filaments Inch pounds 470 0. 450 18 660 0. 465 18 0. 486 46 950 0. 458 38 0. 466 48 980 0.443 l9 0. 453 25 1, 000 0. 450 32 l, 600 0. 462 52 1, 700 0. 460 47 3, 700 0. 460 49 It is further within the scope of the invention to condition filaments prepared from polymers which have been modified by the addition of other substances such as plasticizing agents,resins, cellulose derivatives, or pigments.

Although water in the liquid or vapor stage is I the preferred conditioning agent, it likewise is best gut of the prior art, but their toughness is compensatory in this respect, by reason of the fact that it enables the operator to do the stringing-up to 70 or '75 pounds tension, in the case of tennis rackets-at a faster rate. Comparative tests have demonstrated, moreover, that tennis rackets equipped with filaments conditioned in accordance with the invention outwear those equipped with prior art gut, and by a very wide margin in many instances. They also give playing satisfaction of an order uniformly equal to or greater than that afforded bythe said prior art rackets. Moreover, the new rackets are definitely superior a regards the abrasion resistance of their strings, and their general resistance to weather conditions and to heat.

The several process steps of the invention are generally applicable to. monofils, yarn, thread,

cordage, woven or knitted fabrics, or sheets and ribbons, wherever resistance to sudden or repeated impacts is needed, for instance to lacrosse stick strings, parachute cloth, and certain kinds of sewing threads, but the invention appears to be most uniquely useful for the hereinabove emphasized treatment of large polyamide monofils for use, or used, as strings in tennis, squash, or badminton rackets, and the like. The preferred sizes of filaments for use in tennis and squash rackets are 45 to 60 mils in diameter, and in badminton rackets 25 to 40 mils in diameter.

Since many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

I claim:

1. A process for improving by at least 25% the impact strength of an oriented synthetic linear polyamide filament, which comprises multiple die-drawing said filament to about 46% of its original diameter with at least one-half of the reduction in cross-sectional area of said filament being accomplished before the last die-drawing, subjecting said filament to treatment with a nonsolvent swelling agent therefor, heating said treated filament in a relaxed condition for at least .2 hours while permitting free axial shrinkage of the filamentand terminating said heating prior to a dec'rease in tensile strength of said filament of more than 30%. I

2. The process set forth in claim 1 wherein said polyamide has a melt viscosity at 285 C. of at least 400 poises.

3. A strung racket containing strings comprising oriented synthetic linear polyamide filaments having a high impact strength, said filaments being those obtainable by subjecting the filaments to treatment with a non-solvent swelling agent therefor, heating said treated filaments in a relaxed condition for at least .2 hours while permitting free axial shrinkage of the filaments and terminating said heating prior to a decrease in tensile strength of said filaments of more than 30%.

4. A strung racket containing strings comprising oriented synthetic linear polyamide filaments as set forth in claim 3 wherein said filaments have a diameter between 25 and 60 mils.

5. A strung racket containing strings comprising oriented synthetic linear polyamide filaments as set forth in claim 3 wherein said polyamide ,has a melt viscosity of at least 1000 poises.

6. A strung racket containing strings comprising oriented synthetic linear polyamide filaments ments are those obtainable by multiple die-drawas set forth in claim 3 wherein said polyamide ing the filament to about 46% of its original (11- is polyhexamethylene sebacamide. ameter with at least one-half of the reduction 7. A strung racket containing strings comin cross-sectional area of said filament being acprising oriented synthetic linear polyamide flla- 5 complished before the last die-drawing. ments as set forth in claim 3 wherein said fila- WILLARD E. CA'I'LIN. 

